I had the same issue. And now, thanks to your suggestion it works. I switched from:
m\_toolTip.AddTool(this,ttwtext.c\_str(),cr,1);
To
TOOLINFO ti;
ti.cbSize = sizeof(TOOLINFO);
ti.lpszText = (LPTSTR)last\_ttwtext.c\_str();
ti.hinst = AfxGetInstanceHandle();
ti.hwnd = this->GetSafeHwnd();
ti.uFlags = TTF\_SUBCLASS | TTF\_IDISHWND;
ti.uId = (UINT) this->GetSafeHwnd();
m\_toolTip.SendMessage(TTM\_ADDTOOL, 0, (LPARAM) &ti);
Thanks!